Project Summary Cannabis use poses an important public health problem among emerging adults (EAs), given increasing perceived acceptability and prevalence of cannabis use, combined with state-level legislation resulting in increased access to legal cannabis. Although there are some promising cannabis intervention approaches for college EAs, about half of EAs are not in college. Further, these interventions were developed prior to the legalization of recreational cannabis use in 8 U.S. states and the District of Columbia. In recent years engagement with social media has dramatically increased, with the majority of EAs accessing social media platforms multiple times per day, exposing EAs to online peer influences, which are generally not addressed in existing intervention programs. High levels of engagement with social media content provide a dynamic platform to engage EAs in interventions for cannabis risk reduction. The specific aims of the proposed study are to: 1) iteratively develop and focus test an 8-week social media-based intervention for cannabis use among EAs and 2) test the preliminary efficacy of the intervention versus a 8-week control condition on cannabis- related outcomes. We will use an iterative process to develop a social media-delivered 8-week ?secret? group intervention (i.e., other ?friends? on the site cannot see membership or content) delivered by peer coaches using the most popular social media platform among EAs (i.e., Facebook), with content pushed across other platforms allowing private messaging (e.g., Twitter, Snapchat). Based on prior work, we will develop engaging, user-generated content using crowdsourcing via Amazon's Mechanical Turk, followed by iterative focus testing with participants from states where recreational cannabis is legal and where it remains illegal. The intervention will harness peers as coaches to be positive influences, which is scalable given the availability of student trainees and will focus on ?up-stream? factors for cannabis use such as motives for use (e.g., stress/coping, social), blending evidenced based approaches for cannabis risk reduction (e.g., motivational interviewing, cognitive behavioral therapy strategies). The developed intervention will be tested in a pilot randomized controlled trial versus an attention-placebo control (8-week social media news condition) among 18-25 year-old EAs who regularly use cannabis (i.e., at least weekly). Groups will be conducted separately for EAs residing in states with/without legal recreational cannabis. Outcomes will be measured at a 3-month post-test and 6- month follow-up, with biological verification of drug use. As opposed to static web-based interventions with limited shelf-life, the development of an adaptable, scalable and efficacious intervention for cannabis use among EAs is a critical next step in public health efforts to reduce cannabis use/consequences. This study could have significant impact by altering the cannabis use trajectories of EAs, preventing health and social consequences and the development of cannabis use disorders, as well as informing future social media interventions for behavioral health.